1. Field of the Invention
The present invention relates in general to a method for scanning a keypad architecture of electronic systems. In particular, the present invention relates to the scanning method of keypad architectures utilizing power pin V.sub.DD and ground pin GND of IC devices to minimize the IC pin count while maximizing the number of total keys supported.
2. Technical Background
Schemes for scanning conventional keypads (or, keyboards in certain circumstances) of electronic systems such as telephone sets require the use of a scanning matrix comprising M rows by N columns of scanning signal lines that are handled by the digital circuitry of the system. If an integrated circuit device, herein referred to as IC, of the digital electronic system of a telephone set is responsible for monitoring keypad input, and assuming a M-row by N-column keypad is utilized, then would have to be a total of M+N pins on the IC required to process the scanning signals for the keypad. FIG. 1 shows such a telephone IC 10, and the keypad 20 is monitoring by continuous scanning, using scanning techniques well known in the prior art.
Since the cost of an IC is dependent on the total number of lead pins it has, therefore, an obvious measure which may be used to reduce the cost of the IC used in the electronic system of a telephone set would be to reduce the number of IC pins whenever possible. In a telephone set having a keypad with M-row by N-column inputs, such as is in the case of FIG. 1, wherein M is the number of the scanning rows R.sub.1, R.sub.2, . . . , R.sub.M and N is the number of the scanning columns C.sub.1, C.sub.2, . . . , C.sub.N, in addition to the M+N pins needed for the scanning of the keypad, there are other signals of the telephone electronic system that need to occupy additional IC lead pins.
For example, consider the conventional telephone IC and the keypad it scans, as are schematically shown in FIGS. 2a and 2b. The telephone IC schematically outlined as a pin-assignment layout in FIG. 2a has a total of 18 pins. On the other hand, the keypad it scans has a 4-by-5 scanning matrix. The 4-row by 5-column keypad requires nine IC pins, namely R.sub.1, R.sub.2, R.sub.3 and R.sub.4, as well as C.sub.1, C.sub.2, C.sub.3, C.sub.4 and C.sub.5 as shown in FIG. 2b. These nine scanning pins (and their associated lines) can handle the twelve common keys found on the typical keypad of a telephone set, namely the 0, 1, 2, . . . , 9 keys and the two function keys "#" and "*". In addition, the scanning lines also handle eight other keys, namely the STR, F, AUTO, R/P and M1-M4 keys, which are used for specialized functions.
Of the eighteen pins of the telephone IC in FIG. 2a, nine pins are assigned for the keypad input scanning function and at least two pins are assigned for power and ground connections for the IC. Of the seven remaining pins, two are for the system crystal clock inputs (XT and XT#), one is for mute control (MUTE#), one is for handset status (HKS#), one is for pulse dialing (DP#), one is for audio tone dialing (DTMF), and one is for dialing mode setting (MODE). With these pin assignments, a telephone set can be equipped with the basic functions of a typical telephone system using an eighteen-pin IC. However, when additional telephone set functions are to be added to the system, for example, key-press tone ratio setting, dialing speed setting, speakerphone and/or line hold, etc., additional IC pins are required. This, however, increases the cost of the IC as indicated above.